The present invention relates to an active matrix-type liquid crystal display using an MIM (metal-insulator-metal) device as a non-linear resistance device, and applicable to a high capacitance flat-panel display such as for use in office automation equipment and television sets.
An active matrix-type liquid crystal display generally comprises a liquid crystal layer sandwiched between two insulation substrates, in which a non-linear resistance device is connected in series with an electrode for each of the picture elements disposed on at least one of the substrates. As the non-linear resistance device, an MIM device is often used.
The MIM device known comprises an insulation substrate such as a glass plate; a metal electrode as a lower electrode of Ta, Al, Ti, etc.; an insulator layer of an oxide of the metal described above, SiOx, SiNx, etc. and, further, a metal electrode as an upper electrode of Al or Cr, which are disposed on the insulation substrate in this order.
However, an MIM device using a metal oxide for the insulator layer (refer to Japanese Patent Application Laid-Open (KOKAI) Nos. 57-196589, No. 61-232689, No. 62-62333, etc.) has drawbacks as described below. Since the insulator layer is formed by anodic oxidation or thermal oxidation of the lower metal electrode, production steps are complicated and require high temperature heat-treatment (high temperature heat-treatment is also necessary in anodic oxidation for completely removing impurity, etc.). The film controllability (homogeneity and reproducibility for the film quality, and the film thickness) is also poor. In addition, the materials and characteristics of the device can not freely be changed since the material for the substrate is limited to heat-resistant material and the insulator layer is made of a metal oxide of a certain physical property, to make the degree of design freedom narrow. This means that it is impossible to design and manufacture such a device as capable of satisfying specifications demanded for a device incorporated with MIM devices, for example, a liquid crystal display.
Further, poor film controllability results in the problem that the current to voltage (I-V) characteristic is subject to wide variation. In particular, the symmetry of the current to voltage characteristic (the current ratio I.sub.- /I.sub.+ between a positive bias and negative bias) varies greatly. Moreover, it is desirable that the capacitance of an MIM device is lower in a case of using the MIM device for a liquid crystal display (LCD) since it is necessary that the ratio of liquid crystal capacitance/MIM capacitance is not less than 10. However, since a metal oxide film has a great dielectric constant, the capacitance of the device is also increased, so that precision fabrication is necessary for reducing the capacitance of the device, i.e. the device area. In this case, since the insulator layer suffers from mechanical damages in a rubbing step, etc. upon sealing liquid crystal material, there is also a problem that production yield is lowered, conjointly with the requirement for precision fabrication.
On the other hand, an MIM device using SiOx or SiNx for the insulator layer (Japanese Patent Application Laid-Open (KOKAI) No. 61-275819), is formed by a vapor phase method such as plasma CVD or sputtering. However, since about 300.degree. C. temperature is usually necessary for the substrate in order to deposit the insulator layer, a substrate of low cost can not be used and, in addition, there is the drawback that the thickness and the quality of the film tend to be uneven because of the temperature distribution of the substrate upon increasing the substrate area. Further, the insulator layer is made of amorphous material showing wide variance in the physical properties and, in view of the problem of optical degradation or optical conductivity (resistance change by light), the degree of freedom for the design of the device characteristics is also narrowed.
Further, since a usual MIM device has a sandwich structure, pinholes or voids are often present, particularly, in an insulator layer such as SiNx or SiOx formed by a vapor-phase method and such device defects lower the yield upon mass production (the defect rate increases). Further, in the case of the sandwich structure, since the device characteristics are extremely sensitive to the thickness of the insulator layer, straight control of the film thickness in order to obtain uniform characteristics and, thus, difficult problems are involved in view of the production techniques.
As a result of the present inventors' earnest studies for overcoming the foregoing problems, it has been found that an active matrix-type liquid crystal display using an MIM device comprising electrodes and an insulator layer of a hard carbon film disposed in a structure coplanar to an insulator substrate as a non-linear resistance device can reduce the number of masking operations required in manufacture, reducing the cost as compared with sandwich structures. Our coplanar structures are suitable for inexpensive mass-production and are substantially free from failures caused by short circuits caused by insulation destruction, even if pinholes or voids occur during the manufacture of the device.